1. Field of the Invention
The present invention relates to a machine part, such as a gear for a vehicle, a coil spring, a valve for an internal-combustion engine or a part of a machine, having excellent rolling wear resistance.
2. Description of the Prior Art
Studies of stress relaxing agents, such as sodium benzenesulfonate, paratoluene sulfonamide and saccharin, for relaxing electrodeposit layers have been made and the results of studies have been applied to practical uses. Techniques of Ni--P electroplating machine parts to improve the wear resistance of the machine parts are disclosed in Japanese Patent Laid-open (Kokai) Nos. Sho 61-179899, Sho 63-295855, Sho 63-311624, Hei 1-158211 and Hei 3-18682. Nevertheless, only few studies of the stress relaxation of electrodeposit layers have been made, because Ni--P electroplating has been applied to forming relatively thin electrodeposit layers having a thickness on the order of 10 .mu.m.
The inventors of the present invention have made studies of Ni--P electroplating to improve the rolling wear resistance of machine parts of titanium alloys and aluminum alloys for use particularly as machine parts of automobiles and bicycles. The results of the studies are disclosed in Japanese. Patent Laid-open (Kokai) No. Hei 4-26792. The inventors of the present invention have found through the studies that the Ni--P electrodeposit layers must withstand a very high bearing stress of several hundred kilograms force per square millimeter that will be induced in the machine part due to point contact or line contact to improve the rolling wear resistance of the machine part. The Ni--P electrodeposit layer must be formed in a thickness of several hundred micrometers, which is far greater than the thickness of the Ni--P electrodeposit layer which has been used, to withstand such a high bearing stress. However, if such a very thick Ni--P electrodeposit layer is formed by using a plating bath of a conventional composition under conventional electroplating conditions, namely, current density, the temperature of the plating bath, the pH of the plating bath and so forth, the very thick Ni--P electrodeposit layer will be cracked or will fall off the machine part, and the stress in the electrodeposit layer will break the very thick Ni--P electrodeposit layer when machining the Ni--P electrodeposit layer or when using the machine part coated with the Ni--P electrodeposit layer.
The Ni--P electrodeposit layer coating a machine parts that will be exposed to rolling wear must have a thickness of several hundred micrometers so that the Ni--P electrodeposit layer is able to withstand a high bearing stress of several hundred kilograms force per square millimeter. However, a stress is induced in the Ni--P electrodeposit layer when the Ni--P electrodeposit layer is formed by the conventional Ni--P electroplating process, and the stress induced in the Ni--P electrodeposit layer will cause the Ni--P electrodeposit layer to be cracked, to fall off the machine part or to be broken when the machine part coated with the Ni--P electrodeposit layer is subjected to finishing machining or to a high load.